In recent years, the use of advanced composite structures has experienced tremendous growth in the aerospace, automotive, and many other commercial industries. While composite materials offer significant improvements in performance, they require strict quality control procedures in the manufacturing processes. Specifically, non-destructive evaluation (NDE) methods must assess the structural integrity of composite materials. This assessment detects inclusions, delaminations and porosities. Conventional NDE methods are slow, labor-intensive, and costly. As a result, testing procedures adversely increase the manufacturing costs associated with composite structures.
Various methods and apparatuses have been proposed to assess the structural integrity of composite structures. One method generates and detects ultrasound using lasers. A pulsed laser beam generating ultrasound on a work piece while a second pulsed laser detects the ultrasound. The detection laser, coupled to an interferometer, detects surface displacements or velocity.
In theory, the signal-to-noise ratio (SNR) of a laser-ultrasound detection system should increase as the square root of the light power. However, in practice, as the SNR increases, measurements become more and more sensitive to laser noise. Even when using the quietest single-mode lasers, laser-ultrasound SNR is limited to a maximum value even when detected light power keeps increasing. Experimental results shown in FIG. 1 demonstrate that SNR does not increase when detection laser power increases above 10 mW. Amplitude and phase noises of the detection laser limit the SNR. Being able to overcome this limitation in SNR means that smaller defects could be more easily detected in materials difficult to inspect.
One solution to reject amplitude noise uses a differential interferometric configuration. However this configuration does not reject phase noise. Although SNR improves with this configuration, SNR can become limited by phase noise for large quantities of detected light. Another solution has been to average when SNR is limited by laser noise. However, averaging is slow and SNR increases as the square root of the number of averages. Averaging is therefore an expensive and time-consuming option. A faster and less expensive method to reduce noise and increase SNR is highly desirable.